RIO DE JANEIRO (Reuters) - As if she needed reminding, Kristin Armstrong kept finding little notes with the words "You are a champion" in odd places as she prepared for Wednesday's Olympic time trial.

And so it came to pass on the day before her 43rd birthday as she made light of foul weather on Rio's exotic coast to come out on top in the test of endurance and speed for the third consecutive Games.

But those notes drove her on, especially as they were written by U.S. team mate Mara Abbott, whose failure to win a medal in Sunday's road race when she was caught 200 meters from the line having led for the last 10km was sport at its most cruel.

"I thought about Sunday," Armstrong, who played a supporting roll for Abbott on Sunday and was the first to console her after she finished fourth, told reporters.

"I knew today I had to give it everything for my team and for Mara. The last 24 hours she has been leaving me notes. There was one on my coffee cup yesterday, I found another one on my hairbrush, last night there was one on my pillow.

"And this morning there was one in my podium bag. The support she has given me since her race is phenomenal.

"I feel this was the tightest team I've been on."

Armstrong returned from a second retirement in 2015 to give it one last go for Rio, having also quit the sport in 2009 to start a family. Son Lucas was seen hugging his tearful mom at the finish as the rain beat down.

"Mama, why are you crying?" Armstrong was asked by the 5-year-old who watched as she powered across the finish to win by five seconds from Russian Olga Zabelinskaya.

"That's a great question from a 5-year-old," she said.

"'Why am I crying? Because it's what we do when we're happy!' I'm going to have to explain that one to him a little later."

Armstrong said she still does not know if winning a third gold represented "closure" for her stop-start career.

"I think, of course, any athlete would like to come back and keep coming back, but there does come a point where you won't be able to perform at that level any longer.

"Tomorrow's my birthday and I think I've probably pushed it to the maximum level at the age I am."